The present invention relates to hip prosthesis assemblies for replacing a natural hip socket. More particularly, the present invention relates to an acetabular cup assembly designed to retain a polymeric bearing inside an acetabular metal shell without the use of attachment screws.
It is known to provide an acetabular cup assembly including a metal shell component for attachment to an acetabulum to replace the natural socket and a polymer bearing component which is inserted into the shell to provide a hemispherical bearing surface for receiving a femur ball prosthesis element. Often, the polymer bearing component is nonsymmetrical and includes a built-up lip around a portion of the hemispherical bearing surface to help prevent dislocation of an installed femur ball from the hemispherical bearing surface. During installation of the acetabular cup assembly, the shell component is first secured to the acetabulum. When a surgeon installs the bearing component, the surgeon selects an orientation of the bearing with respect to the shell component to align the lip of the nonsymmetrical bearing component in the most advantageous position to reduce the likelihood of dislocation of the femur ball. Therefore, it is desirable to produce an acetabular cup assembly in which the bearing component can be attached to the shell component in a large number of selected orientations to provide the maximum degree of flexibility for the surgeon. An installed bearing component must be secured to a shell component by a retention force strong enough to prevent dislocation of the bearing component from the shell component.
One object of the present invention is to provide a retention mechanism for retaining a bearing component situated at any selected orientation inside a shell component which does not rely on the physical properties of the bearing component.
Another object of the present invention is to reduce loading of forces on an outer lip or flange of the bearing component after the hip prosthesis is installed in a patient.
Still another object of the present invention is to prevent rotation of the bearing component relative to the shell component after insertion of the bearing component into the shell component in a desired orientation.
A further object of the present invention is to provide a self adjusting locking mechanism which retains the bearing component inside the shell component despite possible shrinkage of the bearing component after installation.
According to the present invention, a prosthetic acetabular cup assembly is provided for receiving a ball attached to a femoral prosthesis. The assembly includes a bearing component having an inner bearing surface for receiving the ball and an outer surface. The assembly also includes a shell component for attachment to an acetabulum to replace a natural hip socket. The shell component includes an inner surface defining a cavity for receiving the bearing component therein. The inner surface of the shell component is formed to include an arcuate groove therein. The assembly further includes means located in the arcuate groove of the shell component for engaging the bearing component to retain the bearing component inside the shell component upon insertion of the bearing component to the shell component.
In one preferred embodiment of the present invention, the bearing component includes an arcuate groove formed in the outer surface. The arcuate groove of the bearing component is positioned on the outer surface of the bearing component in a position axially aligned with the arcuate groove of the shell component upon full insertion of the bearing component into the shell component. The engaging means includes a formed wire located inside the arcuate groove of the shell component. The wire is configured so that a portion of the wire extends radially inwardly from the arcuate groove of the shell component to lie within the arcuate groove of the bearing component. Therefore, the wire engages the bearing component to retain the bearing component inside the shell component. In an alternate embodiment, an expandable lock ring can be used in place of the wire to secure the bearing component to the shell component.
One feature of the present invention is the provision of a lock wire or lock ring situated inside an arcuate groove formed in the inner surface of the shell component. The shell component is typically made from metal such as titanium. Therefore, the position of the wire or lock ring is accurately maintained during insertion of the bearing component into the shell component. This prevents shifting of the axial position of the wire or lock ring relative to the shell component and ensures that the wire or lock ring will be properly aligned with the arcuate groove formed in the outer surface of the bearing component to retain or "lock" the bearing component in place upon insertion of the bearing component into the shell component.
Another feature of the present invention is the provision of anti-rotation lugs formed on the inner surface of the shell component. The lugs are situated below the arcuate groove formed in the inner surface of the shell component. The lugs interfere or machine into the outer surface of the bearing component as the bearing component is inserted into the shell component to prevent rotation of the bearing component relative to the shell component. There are no preformed slots in the bearing component for receiving the lugs. This feature advantageously provides no clearance or tolerance between the lugs and the outer surface of the bearing. Torsional backlash can occur if there is any clearance or tolerance between the lug and a preformed notch in the bearing component. Torsional backlash occurs when a force is exerted on the bearing component after installation of the assembly into the patient which causes slight movement of the bearing component. Therefore, the present invention reduces torsional backlash by providing no clearance or tolerance between the lugs and notches formed in the bearing.
Yet another feature of the present invention is that the locking mechanism permits "infinite dialability" of the bearing component with respect to the shell component while maintaining its ability to lock the bearing component inside the shell component. Bearing components are often nonsymmetrical and include a lip portion which aids in the retention of a femur ball within the bearing component. The locking mechanism of the present invention advantageously permits the surgeon to align the bearing component at any one of a continuum of positions relative to the shell component and then lock the bearing component in that position. This gives the surgeon greater flexibility when installing the assembly and allows the surgeon to select the precise orientation of the bearing component which is most advantageous to the patient.
Still another feature of the present invention is the provision of a shell component having an inner surface which is congruent with an outer surface of the bearing component. Advantageously, this congruency prevents loading on the lip or flange surrounding the bearing component. Such lip loading can occur when the femur ball exerts forces on the bearing component and the bearing component is not congruent with the shell component.
A further feature of the present invention is the provision of a locking mechanism which is capable of adjusting itself to compensate for changes which can occur in the bearing component. Because the bearing component is made from a polymeric material, its size can change over time. Often, bearing components show a propensity to shrink slightly over time after installation. Advantageously, the lock ring or wire of the present invention directs a retaining force radially inwardly into the cavity from the inner surface of the shell component and toward the bearing component. Therefore, if the arcuate groove in the bearing gets deeper due to wear over time or the bearing component shrinks after installation, the lock ring or wire can move radially inwardly to compensate for such changes in the bearing component.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.